An NIH study published on October 2 in the online edition of Immunity suggests that the genetic material of bacteria living in the intestines of humans may function in fighting infection. The findings expand the known role of gut bacteria, or commensals, which include aiding in digestion.

It is normal for a person to have anywhere from 300 to 500 different species of bacteria living in their intestines. The relationship between two species can be classified into three main groups: parasitism, in which one species benefits at the expense of the other; commensalism, in which one species benefits and the other is not harmed; and mutalism, in which both species benefit. It is well-established that bacteria live in the gut without causing any harm to humans, and for years researchers have categorized this relationship as commensal. However, recent research seems to be characterizing a more mutualistic interaction.

"Within the body of a healthy adult, microbial cells vastly outnumber human cells. Research to understand these microbial communities is an exciting scientific frontier," says Dr. Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases (NIAID). "Among many opportunities related to the so-called microbiome,' targeting beneficial bacteria may offer new avenues for therapy against infectious and immune-mediated diseases."

The exact mechanism of how friendly bacteria help ward off pathogens isn't known. However, the report indicates the gut microbes bind to the body's disease-fighting cells known as T-cells in order to boost their activity.

"There is a balance of regulatory immune signals in the body," notes Dr. Yasmine Belkaid, lead investigator of the study. "During an infection, we've found that commensals can break this balance in favor of an infection-fighting response."

In healthy individuals, intestinal T-cells known as Tregs regulate the commensals and guard them from immune system attack. Dr. Belkaid's team found that during an infection the commensals will bind to a specific site on the Tregs, gearing them into attack mode. The Tregs then specifically seek out the pathogens and ward off the invasion.

The team is excited about the findings because they may lead to a better understanding of how to go about treating autoimmune disorders such as rheumatoid arthritis and Stokes disease. The harmonious relationship between gut microbes and humans illustrates the perfect paradigm for synthetic treatments: protect the good, attack the bad.